Pyridoxal phosphate (PLP) is involved in a greater variety of enzymatic processes than any other coenzyme. PLP dependent enzymes participate in amino acid metabolism in the formation and degradation of neuro-transmitters and other biogenic amines of the central nervous system and are the target of PLP, analogues as antitumor agents. Even though the enzymes require PLP, they differ in the mode of binding of the coenzyme and utilization of the initial covalent enzyme-substrate complex. We intend to study two PLP representative enzymes of mammalian origin, aspartate transaminases. We intend to determine: The properties of the phosphate binding site; the values of the pK of the active site amino acids, the detection of new enzyme-substrate complexes, the properties of substrates and inhibitors when bound to the enzymes, and probe the microheterogeneity among the enzyme subunits. The experimental methods utilize: Formation of stable F19 labeled substrate-coenzyme complexes, P31 nuclear magnetic resonance (nmr) of the enzyme bound phosphate, F19 nmr of the chemically modified enzyme or F19 labeled substrates, and stopped-flow and equilibrium kinetics of native and chemically modified enzymes.